This invention relates to pilot protective relays and more particularly to the communication of status and waveform information from two ends of a protection zone.
As is well known pilot protective relays are used to provide protection for high-voltage electrical transmission lines. Those sections of the transmission lines that have pilot protective relays located at each end are known as the protection zones. It is the function of the pilot relays to identify faults in the protection zone and initiate line isolation by way of the tripping of the appropriate circuit breakers. To provide this protection the pilot protective relays monitor the AC voltages and currents at the transmission line terminals and coordinate operation by communicating status and waveform information from the two ends of a protection zone. Generally, such differential protection performance improves as the amount of information increases.
As used herein waveform information refers to the analog information about the currents and voltages at the transmission line terminals and any digital representation of that analog information, and status information refers to information which is other than waveform information and is always in digital form. Status information is usually a single bit or a very small number of bits for each such item of information. As is well known voltage and current sensors, each known as a channel, are used to gather the analog waveform information. Each sensor includes an analog to digital converter which converts the analog current or voltage waveform information to a digital signal with a multiple number of bits.
The communication distances between the two ends of a protection zone may be many kilometers. The prior art has employed a variety of schemes with pilot protective relays to communicate the status and waveform information from the two ends of the protection zone. These schemes include wireline with simple on-off keying audio tone modems, wireless and fiber optic techniques.
The demands of the protective relays require that the status and waveform information be delivered in a timely fashion since the protective function should be able to make trip decisions in a time period that is less than one cycle of the power line frequency. Thus the trip decisions must be made in less than 16 milliseconds if the power line frequency is 60 Hz and in less than 20 milliseconds if the power line frequency is 50 Hz.
Present day communication of the status and waveform information from two ends of a protection zone by using techniques such as wireline in the form of, for example, plain old telephone service, currently utilize 9600 baud modems with xc2xd cycle or less latency. Using more sophisticated protocols such as V.90 is not feasible because of delays associated with such modems. The delays give rise to a delivery time that cannot be guaranteed and which is not acceptable if trip decisions are to be made in a time period that is less than one cycle of the power line frequency. Thus, common practice dictates jumping to fiber optic channels with 56 KB bandwidth to accommodate desired information transfer with minimum latency and variability of delivery as the use of such channels allows for trip decisions to be made in a time period that is less than one cycle of the power line frequency.
The techniques presently in use in wireline modem communication at 9600 baud rely on xe2x80x9cCompromisedxe2x80x9d not xe2x80x9cCompressedxe2x80x9d data. For instance, a single weighted assembly of a symmetrical component computation is transmitted instead of the individual components. A phasor is sent as only one component (Real or Imaginary) and the other is deduced using the same waveform delayed xc2xc cycle (a close approximation). As few as four samples per cycle are transmitted due to bandwidth limitations, whereas the actual waveform sample rate is considerably more than four samples per cycle. These are just a few of the lossy xe2x80x9ccompromisesxe2x80x9d in data transmission accommodating the method and media chosen for the communication link.
It is possible to increase the information data rate for the same signaling baud rate by utilizing known characteristics of the data to be transmitted. This leads to xe2x80x9cInformation Dependent Transmissionxe2x80x9d. Signals are relatively stationary in normal operation and a simple expedient of sending the xe2x80x9cdifference from the normxe2x80x9d may increase the information throughput for the same signaling bandwidth. The signals are, however, not relatively stationary in abnormal operation and the simple expedient of sending the xe2x80x9cdifference from the normxe2x80x9d will not increase the information throughput for the same signaling bandwidth. Thus it is desirable to have a high capacity pilot protective relay to pilot protective relay data transmission scheme using wireline modem communications not only at 9600 baud but also at other transmission rates. It is further desirable to have such a scheme which uses information dependent data transmission. It is also further desirable to have such a scheme that improves information throughput independent of the transmission technique.
In addition, the high capacity data exchange scheme should be useable by those electric utilities that follow the well known IEEE Standard Common Format for Transient Data Exchange (COMTRADE) for power systems in keeping records of fault data for post-fault analysis such as determining the nature and location of the fault. Further details about the COMTRADE standard can be found in the xe2x80x9cIEEE Standard Common Format for Transient Data Exchange (COMTRADE) for Power Systems,xe2x80x9d IEEE C37.111-1991, IEEE Standards Board, Approved June 1991.
The present invention is a method for an electrical transmission system. The system has at least one pilot protective relay at a first location and at least one pilot protective relay at a second location separated from the first location. The method is for transmitting waveform information from the first location to the second location and from the second location to the first location. The method comprises has the steps of:
a) compressing the waveform information at the first and the second locations using a lossy compression technique; and
b) compressing the lossy compressed waveform information at the first and second locations using a loss-less compression technique.
The present invention is also a method for an electrical transmission system. The system has at least one pilot protective relay at a first location and at least one pilot protective relay at a second location separated from the first location. Each of the at least one pilot protective relays having waveform information associated therewith. The method is:
a) compressing the associated waveform information at the at least one pilot protective relay at the first location and at the at least one pilot protective relay at the second location; and
b) transmitting the compressed associated waveform information from the at least one pilot protective relay at the first location to the at least one pilot protective relay at the second location and from the at least one pilot protective relay at the second location to the at least one pilot protective relay at the first location.
The invention is further an electrical transmission system. The system has a pilot protective relay at a first location on the system and a pilot protective relay at a second location on the system. The pilot protective relay at the first location transmitting waveform information from the first location to the second location and the pilot protective relay at the second location transmitting waveform information from the second location to the first location. The system also has means for lossy compressing the waveform information at the first and second location; and means for loss-less compressing the lossy compressed waveform information at the first and second locations.